The anode of a typical solid electrolytic capacitor consists of a porous anode body with a lead wire extending beyond the anode body and connected to the positive mounting termination of the capacitor. The anode is formed by first pressing a valve metal powder into a pellet. Alternatively, the anode may be an etched foil, for example aluminum foil as is commonly used in the industry. Valve metals include Al, Ta, Nb, Ti, Zr, Hf, W, and mixtures, alloys, nitrides, or sub oxides of these metals. NbO may also be used as an equivalent to a valve metal. The pressed anode is sintered to form fused connections between the individual powder particles. All anodes are anodized to a pre-determined voltage in a liquid electrolyte to form an oxide of the valve metal which serves as the dielectric of a solid electrolytic capacitor. A primary cathode material, such as a conductive polymer or manganese dioxide, is subsequently applied via a multi-cycle liquid dipping process. In order to minimize the equivalent series resistance (ESR) of solid electrolytic capacitors the devices subsequently are dipped in a silver paint, which when dried provides a highly conductive cathode terminal coating. A carbon layer, usually applied between the primary cathode material and terminal silver layer, serves as a chemical barrier to isolate the two layers. The silvered anodes are then assembled and encapsulated to form the finished devices. The encapsulation process may be a transfer molding process or conformal coating process to manufacture surface mount capacitors. Conformal coating with a plastic sealant is often used to manufacture leaded devices. The industry standard case sizes for surface mount capacitors are rectangular solids, thus rectangular anodes or parallopipeds are used in these devices to maximize volumetric efficiency. In hermetically sealed capacitors the silvered anodes are placed in cylindrical cans containing a solder plug. Heat is applied to the can to reflow the solder. After reflow the solder secures the anode in place and forms an electrical connection between the cathode and the metallic can. The anodes used in these devices are cylindrical.
The reliability of all such devices is highly dependent on the quality of the external cathode layers.
The ability to isolate flaws in the dielectric is a requirement of the primary cathode material chosen for manufacturing solid electrolytic capacitors. This property of the primary cathode material results from a process termed ‘healing’. The application of voltage to the capacitor causes current to flow through flaw sites in the dielectric, resulting in an increase in the temperature at the defect site due to Joule heating. As current flows through the flaw site the counter electrode material immediately adjacent to the flaw site is rendered nonconductive. The temperature of the cathode layer immediately adjacent to the flaw site also increases due to conduction. When manganese dioxide is employed as the cathode material, the manganese dioxide immediately adjacent to the flaw site is converted to manganese sesquioxide at the decomposition temperature of manganese dioxide (500-600° C.), thus isolating the flaw. Since the resistivity of manganese sesquioxide is several orders of magnitude greater than tha1t of manganese dioxide, leakage currents through the flaw sites decrease according to Ohm's law. A similar mechanism is postulated for conductive polymer counter electrodes. Possible mechanisms to account for the healing mechanism of conductive polymer films include complete decomposition of the polymer adjacent to the flaw site, over oxidation of the polymer, and dedoping of the polymer at the flaw site. At temperatures above 600° C. the amorphous tantalum pentoxide which serves as the dielectric in tantalum capacitors is converted to a conductive crystalline state. Thus, in order to be an effective primary cathode material for tantalum the material must convert to a nonconductive state at temperatures below 600° C. The maximum withstanding temperatures of other valve metal oxides is similar to that of tantalum.
Since the graphite and silver layers do not decompose to form nonconductive materials at temperatures below 600° C., continuous coating of all dielectric surfaces by the primary cathode material is essential to prevent the graphite or silver layers from contacting the dielectric. If the graphite or silver do contact of the dielectric the device there will be a short circuit.
Conductive polymer coatings are applied to the anode using a variety of methods as described in U.S. Pat. No. 6,072,694. The use of polymer slurries or liquid suspensions containing pre-polymerized conductive polymer as an alternative to the monomer is very attractive due to the simplicity of manufacturing, the reduction in waste, and the elimination of costly and time consuming washing steps after each coating step as directed in U.S. Pat. No. 6,391,379. Although this process approach is attractive it has not yet been implemented on a production scale. One of the principle technical obstacles to the successful implementation of a polymer slurry to serve as the primary cathode layer is the difficulty coating edges and corners of the anode with slurry. These materials tend to pull away from corners and edges due to surface energy effects. The resulting thin coverage at corner and edges results in reduced reliability of the device. The magnitude of the force pulling the liquid away from the edge is given by the Young and Laplace Equation:Δp=γ/r Wherein
Δp=the pressure difference causing the liquid or slurry to recede from an edge
γ=the surface tension of the liquid or slurry; and
r=the radius of curvature of the edge.
This effect is illustrated in FIG. 1.
During dipping the liquid phase of a suspension will enter the pores of the anode. If the particles in the suspension are larger than the pores, they will be prevented from entering the anode body and can buildup on the external surface of the anode. Thus external buildup on the faces and corners of the anode after dipping in the slurry is somewhat dependent on the void volume (i.e. density) of the anode. Variations in local density of the anode can result in non-uniform coating, especially on the corners and edges of an anode.
The reliability of a solid electrolytic capacitor is also degraded due to differences in coefficients of thermal expansion between the anode bodies and encapsulates material. These mismatches lead to thermo-mechanical stresses on the anode/dielectric during surface mounting. These stresses are greatest at the edges and corners of the anode body. Capacitor manufacturers rely on the external cathode coatings of carbon and silver paint to reduce or distribute the stress, especially at high stress points like corners and edges of the anode. However, the external cathode layers often are applied in the form of liquid slurries or suspensions which produce thin coverage at corners and edges resulting in reduced reliability of the device.
Capacitor manufacturers have also employed rectangular prism anode designs with designated rounded or chamfered edges in order to reduce the thermo-mechanical stress on edges of surface mount devices after encapsulation. An anode with chamfered edges at the top of the anode was described by D. M. Edson and J. B. Fortin in a paper published in the Capacitor and Resistor Symposium in March 1994 entitled “Improving Thermal Shock Resistance of Surface Mount Tantalum Capacitors.” These authors used finite element analysis and failure site identification techniques to demonstrate that most failures which occurred during surface mounting were along the top edges of the anode (surface where the lead projects). A modified anode design as depicted in FIG. 2 was reported to reduce the surface mount failure rate.
An anode with a chamfered portion at the top edge of the anode (see FIG. 3) is described in U.S. Pat. No. 5,959,831 (Maeda, et al.). The purported purpose of this design is to reduce the likelihood of the primary cathode layer wicking up the anode lead wire during dipping. In U.S. Pat. No. 7,190,572 the inventor claims that excess edge buildup of manganese dioxide can be avoided by chamfering the bottom edges of the anode (see FIG. 4). The buildup of manganese dioxide at the corners is the opposite phenomenon to that observed when conductive polymers are applied. Also, some rounding of side edges of pressed anodes has been observed in capacitors on the market. (see FIG. 5).
One of the drawbacks of rounded side edges as depicted in FIG. 5 is the difficulty in pressing reproducible anodes with these geometries using a radial press design. A radial press design is defined as a press which compacts the powder in a direction perpendicular to the anode lead wire (typically the long axis of the compact). Axial presses are defined as a press which compacts the powder in a direction parallel to the anode lead wire. Although axial pressing allows for greater flexibility in anode geometries of interest to capacitor manufacturers, it often leads to other problems such as smearing of the powder at the surface as it slides inside the die cavity and density gradients in the anode in the axial direction of the anode lead. The high density regions and powder smearing make it more difficult for the liquid phase of a slurry or suspension to enter the pores of the anode, exacerbating the problems of poor cathode coverage. Although powder smearing and density gradients also occur with radial pressing, they occur to a considerably lesser degree since the longest dimension of the anode is typically along the length of the anode lead wire.
Although rounded and chamfered edge rectangular anodes have been described and utilized in the industry for years, the concept of corner chamfering has not been explored. In fact, since the edges represent a line drawn between two points, the corners, corner rounding would not be expected to provide benefits beyond that of edge rounding. However, analysis of many electrically failed conductive polymer anodes indicates that the dielectric breakdown mainly occurs on the corners of the anodes, as shown in FIG. 6.
Another approach to improving corner coverage would be to eliminate the corners through the use of cylindrical or obround anode geometries. However cylindrical anodes are volumetrically inefficient when used in industry standard case dimensions for surface mount product. Obround anodes are more volumetric efficient, but pressing these anodes is generally done on an axial style press. This leads to density gradients and high densities at either the top or bottom edge of the anode. These high density edges are as difficult to fully coat with a slurry as a corner on a rectangular shaped anode.
Axial leaded hermetically sealed solid electrolytic capacitors are extremely reliable capacitors. Because the only heat introduced in the solder attach process is to the leads, on the opposite side of the printed circuit board (PCB) as the part, the temperature rise is small, and damaging forces (mismatch in coefficients of thermal expansion) created by this process are minimal. Compared to the forces created in the solder process for surface mount capacitors (SMT) where the entire capacitor package is immersed into the high thermal profile of the solder, theses forces should never create failures. This fact is born out in the recommended applications of these capacitors. Leaded capacitors may be used up to 80% of its nameplate voltage, whereas the product is limited to 50% of its nameplate voltage.
The big disadvantage for these leaded products is the susceptibility to mechanical forces created in handling of the parts. As loose pieces are handled, there is a potential of dropping the device, crimping the device, or pressing on the device in the process of moving from package to place in circuit that is not detectable. If the piece survives the initial electrical testing, the flaw created by the physical force can grow and become a circuit failure at a later point.
Through diligent research the present inventors have found that polymer coverage and leakage are improved by various techniques of modifying the corners of anodes to improve the corner coverage with the primary cathode material. Problems of poor edge coverage on obround or cylindrical anodes can be overcome by modifying the edges of these anode designs. The reliability of hermetically sealed devices can be improved by similarly modifying the edges of cylindrical anodes used in this style capacitor.